The present invention relates generally to electronic circuit tuning, and more particularly to pulse-width modulating capacitors to achieve low-noise, fine-frequency tuning.
The ability to finely tune a frequency response or characteristic of an electronic circuit while maintaining low noise is very desirable in applications such as wired or wireless switches, controllers, and transceivers, filters, power management, data storage, and others. The pursuit of this goal has taken many forms: each with limited success.
For example, crystals can be used where high accuracy and low temperature drift is needed. But these devices cannot be included on an integrated circuit. As a separate device, it requires the use of at least one integrated circuit pin and consumes system board space.
Varactor diodes have the advantage that they can be included on an integrated circuit. But when the capacitance of a varactor diode is tuned using an analog voltage provided in an open loop manner, the resulting noise can create difficulties. The current in the circuit that generates the control voltage can be increased in order to reduce the noise, but this is very undesirable, particularly in battery powered electronic systems.
Digitally adjusted capacitors (capacitors that are either connected or disconnected using a switch) can be used to improve on this noise issue. Digitally adjusted capacitors are typically either equally or binarily weighted. Where they are equally weighted, a large number of capacitors and corresponding switches are required to provide tuning having a large tuning range with a fine resolution. Where they are binarily weighted, the smallest capacitors are limited in size by their switches' parasitic capacitances, while the largest capacitors are limited by die area considerations. Thus, once again, it is difficult to finely tune the circuit's response or characteristic.
Thus, what is needed are circuits, methods, apparatus, and code that provide low-noise and high-resolution tuning for electronic circuits.